1. Field of the Invention
The invention is directed to a tube bundle reactor for carrying out catalytic gas phase reactions, particularly methanation reactions, having a bundle of catalyst-filled reaction tubes through which reaction gas flows and around which heat carrier flows during operation, wherein, in the region of the catalyst filling, the reaction tubes run through at least two heat carrier zones which are separated from one another, the first of which heat carrier zones extends over the starting region of the catalyst filling, and wherein during operation the heat carrier temperatures for each heat carrier zone can be adjusted in such a way that they decrease from zone to zone in flow direction of the reaction gas.
By “starting region of the catalyst filling” is meant within the framework of the present application the region in the catalyst filling in which a region of maximum reaction temperature, or hot spot, develops shortly after the entrance of the reaction gas.
2. Description of the Related Art
In the face of finite natural gas deposits, there is a growing interest in the production of a natural gas substitute. This gas, known as SNG—substitute natural gas or synthetic natural gas—is produced from coal, particularly lignite coal, or biomass (bio-SNG or biomethane) by way of synthesis gas. In order for SNG to substitute for natural gas, the composition and properties of the SNG must correspond as far as possible to those of natural gas. For injection of SNG into the natural gas grid, for example, the “DVGW G260/G262 Specification on Substitute Gas”, referred to hereinafter as “injection specification”, states that the CO2 content may be no more than 6 vol % and the H2 content no more than 5 vol %.
Irrespective of feedstock, SNG production is carried out in four process steps. First, the carbon-containing feedstock is broken down by addition of heat, water and possibly oxygen to form a synthesis gas which is chiefly composed of carbon monoxide, carbon dioxide, hydrogen and water. Injection of air is avoided on principle because the main constituent thereof; nitrogen, limits the methane concentration and undesirable side reactions can take place at higher temperatures. Synthesis gas production is carried out either in an autothermal reactor in which the heat is generated through partial oxidation by the reaction gas itself or in an allothermal reactor in which heat is supplied externally. The latter method has the advantage that additional flue gas is not introduced into the process gas. In a second step, the synthesis gas is cleaned and cooled. The actual methanation is carried out in a third step. In a fourth step, undesirable gas constituents are removed from the synthesis gas and its composition is adapted to the desired application.
The essential methanation reaction is strongly exothermic. It is catalyzed with elements of main group VIII, preferably nickel. In so doing, 3 moles of hydrogen react with one mole of carbon monoxide and 4 moles of hydrogen react with one mole of carbon dioxide to form methane and water:CO+3H2CH4+H2O ΔHRO=−206 kJ/moleCO2+4H2CH4+2H2O ΔHRO=−165 kJ/mole.
Since the large proportion of hydrogen for a complete conversion is usually not present in the original synthesis gas, it is often increased by converting carbon monoxide and water to hydrogen and carbon dioxide using the so-called water gas shift reaction according to the following equation:CO+H2OH2+CO2 ΔHRO=−41 kJ/mole
A multitude of other side reactions also take place in addition, including the Boudouard reaction in which carbon monoxide decomposes to carbon and carbon dioxide:2COC+CO2 ΔHRO=−173 kJ/mole
This reaction is undesirable because the carbon monoxide needed for methanation is removed and the carbon settles on the surface of the catalyst and deactivates it. This reaction can be suppressed through addition of water.
The article “Development of a method for methanation of biomass-based synthesis gas in honeycomb catalysts, GWF, 150 (2009) Nos. 1-2, pp 45-51, discusses the equilibrium conversion of the methanation reaction under stoichiometric conditions. According to this article, methane formation is promoted at high pressures and low temperatures. In pressureless operation, CO conversions of up to 95% are possible in a temperature range between 200° C. and 350° C. At higher temperatures, the pressure must always be further increased to obtain identical conversions. Here also, the reaction rate climbs as the temperature increases. In pressureless operation, a residence time of 1 s is required for a virtually complete conversion at a temperature of approximately 225° C., and a residence time of 0.01 s at about 325° C. Thus the reaction conditions have a decisive influence on the reactor size and reactor concept.
The article “SNG from coal and dry biomass—A technology review from 1950 to 2009”, Fuel, Volume 89, Issue 8, August 2010, pp 1763-1783, gives a broad overview of the methods developed for the production of SNG.
A large number of methods were developed at the time of the oil crisis in the 1970s with the focus on producing SNG from readily available coal. Methods using fixed bed reactors are known as the Lurgi, TREMP (Haldor Topsoe), Conoco/BGC, HICOM, Linde, RMP and ICI/Koppers processes. Fluidized bed methods were developed by the Bureau of Mines, in the Bi-Gas Project and in the Comflux process. Other concepts include the Synthane Project, catalytic coal gasification and liquid phase methanation.
The reactors employed work either adiabatically with intermediate coolers or are preferably cooled by boiling water. Often, a partial flow of the product gas is cooled intermediately and returned to the reactor input.
So far, the world's only industrial plant for methane production from coal is the Great Plains Synfuels Plant in North Dakota, USA, which has been in operation since 1984. Further plants were not built due to the high investment costs and discovery of new natural gas sources. In the North Dakota plant, synthesis gas is generated in 14 parallel gasifiers and is methanated in a number of autothermal methanation reactors with intermediate cooling stages. The carbon dioxide occurring in so doing is stored underground.
German Published Application No. 25 49 439 describes a method for producing SNG in which a preheated synthesis gas containing carbon oxides and hydrogen is conducted through an adiabatic fixed bed reactor and a portion of the product gas is recycled and mixed with the raw gas. Returning the hydrogen-containing product gas renders a shift reaction superfluous. On the other hand, the size of the reactor is increased slightly. The inlet temperature is between 250° C. and 350° C., the outlet temperature is between 500° C. and 700° C. The product gas is cooled so that it has a temperature between 250° C. and 350° C. and is at least 50 degrees above the dew point. A portion of the product gas is extracted as cycle gas and is returned to the raw gas flow.
A method for generating a gas with a methane content of from 90 to more than 98 mole % is described in U.S. Pat. No. 3,922,148 and in the corresponding German Published Application No. 25 21 189 A1. Here, high boiling fractions of the petroleum preparation are first converted with pure oxygen and steam to synthesis gas with a methane content of 6 to 20 mole %, dry, at temperatures between 650 and 930° C. and a pressure between 25 and 150 bar. After cooling and cleaning, the methane content of the gas is increased stepwise in three fixed bed methanation reactors to the final concentration. The gas is cleaned and cooled between stages in each instance. The process is very cumbersome due to the large number of methanation steps. Further, a portion of the feedstock must be burned in generating the synthesis gas. Large amounts of working steam are generated. On the whole, the efficiency of the process is poor.
U.S. Pat. No. 3,970,435 describes a methanation reactor with synthesis gas supplied from coal gasification. The synthesis gas coming from the gasifier has a composition of 35% H2, 35% CO, 2% CO2, 2% CO, 2% H2O, 25% CH4 and 1% impurities, e.g., H2S, flyash or tar. The pressure is approximately 69 bar. The hydrogen content is increased in a shift reactor. After a CO2 gas scrubbing, the gas contains 56.2% H2, 18.8% CO and 25% CH4. The temperature at the inlet into the first reactor is 260° C. It comprises a boiling-water tube bundle reactor with catalyst-coated star-shaped inserts in the reactor tubes. The catalyst inserts separate the reaction tube cross-sectionally into a plurality of independent flow channels. This construction results in low pressure losses. The activity of the catalyst varies along the tube axis and is adjusted in such a way that preferably the same amount of heat is generated along the entire length of the tube. The lifetime of the catalyst is increased in this way. The reaction tubes are assembled in rows which are immersed in boiling medium, preferably water, in a plurality of receptacles. Return lines are additionally provided inside each receptacle. The pressure in the heat carrier space is limited to approximately 69 bar corresponding to a boiling temperature of water of 285° C.
European Published Patent Application No. 2 2110 425 proposes a similar multiple-stage process with the important difference that the methanation reactors which are connected in series operate adiabatically and each have a downstream product cooler. The feedstock for synthesis gas generation is either coal or biomass. The prior art proceeds from a multiple-stage process in which large amounts of product gas are fed back to the respective individual reactors as cycle gas to thin the inlet gas entering the reactor. According to European Published Patent Application No. 2 110 425 A1, the amounts of cycle gas are reduced in that the synthesis gas is not returned in its entirety to the first reactor but is rather distributed parallely to the individual reactors. Cycle gas is used only in the first reactor. Prior art methanation reactors are used. The levels of catalyst activity in the individual reactors is not discussed. The operating pressure is 35 bar, the inlet temperature is between 240° C. and 300° C., the outlet temperature is about 600° C.
German Published Patent Application No. 24 36 297 describes a process for producing a SNG which can be injected directly into the natural gas grid. The raw gas that is used is produced through gasification under pressure of coal, tars or heavy residual oils with the addition of steam and oxygen at 1100° C. to 1500° C. under pressures between 20 and 80 bar. After cooling and cleaning, particularly of sulfur compounds, it is subjected to methanation in at least two stages over nickel catalysts under pressures from 5 to 100 bar and temperatures between 200 and 500° C. After the first methanation stage, the product gas has a methane content of at least 60 vol % and is converted in the last methanation stage over a catalyst that is cooled indirectly with gas countercurrently.
German Published Patent Application No. 27 29 921 A1 presents a process for generating a SNG with at least 80 vol % methane which is carried out by multiple-stage methanation in fixed beds with nickel catalysts under pressures of 5 to 100 bar. In so doing, a synthesis gas is first produced, cleaned and cooled according to a known method for coal gasification under pressure. With a methane content of 8 to 25%, it is first converted in a high-temperature methanation with inlet temperatures of between 230° C. and 400° C. and outlet temperatures between 550° C. and 750° C. and subsequently guided in a low-temperature methanation at temperatures from 230 to 500° C. The high-temperature methanation is carried out in at least two adiabatic fixed bed reactors. In so doing, a portion of the product gas exiting from the fixed bed reactors is removed, cooled, condensed and fed back to the respective reactor inlet. The methane-containing product gas exiting from the final high-temperature fixed bed reactor is cooled before entering the low-temperature methanation. The low-temperature methanation is carried out in an adiabatic fixed bed reactor and a cooled countercurrent flow reactor, e.g., according to German Published Patent Application No. 24 36 297.
In German Published Patent Application No. 29 49 588, a methane-rich gas is produced at elevated pressure and elevated temperature in such a way that the supplied synthesis gas is divided into two partial flows. The first partial flow is guided through a catalytic adiabatic fixed bed and is subsequently cooled to 250° C. to 400° C., and superheated steam is generated by which steam turbines can be driven for generating power. It is subsequently mixed with the second partial flow and guided into a second methanation reactor which, however, is cooled. A shift reactor is arranged immediately in front of the first adiabatic fixed bed reactor.
United States Published Patent Application No. 2010/162626 claims an adiabatic methanation reactor with two reaction zones which are filled with catalyst and separated from one another by a separating wall. The reaction gas mixture is divided into preferably two equal partial flows and fed to the reactor on opposite sides so that the two partial flows are guided through the reactor countercurrently. In this way, the reaction gas is cooled at the end of a reaction zone by the reaction gas entering on the other respective side of the separating wall. The reactor is preferably constructed as a tube bundle reactor. In a known manner, the first portion of the respective catalyst charge is used for the shift reaction and the second portion is used for the actual methanation. This first reactor is followed by further strictly methanating reactors which are constructed in the same way as the first methanation reactor. There are cooling stages between the individual reactors.
In German Published Patent Application No. 32 47 821, a reaction tube filled with catalyst is used for a methanation reaction, its first section having a reduced hydraulic diameter. The entire reaction tube is preferably cooled by boiling water.
International Published Patent Application No. 03/011449 discloses a tube bundle reactor with reaction tubes whose hydraulic diameter increases conically in uniform steps or continuously in the flow direction of the reaction gas. The heat carrier flows counter to the flow direction of the reaction gas from the larger end of the reaction tube to the smaller end of the reaction tube. Further, it is suggested to arrange the reaction tubes next to one another in groups and to enlarge the hydraulic diameter from group to group or to form all reaction tubes identically but increase the quantity thereof from group to group. In so doing, one and the same heat carrier stream flows through all of the reaction tube groups, or each reaction tube group has its own heat carrier circulation as in the adjacently arranged individual tube bundle reactors.
European Published Patent Application No. 1 627 678 A1 suggests a method for carrying out catalytic gas phase reactions using a tube bundle reactor in which the reaction gas mixture is divided up at the inlet into the reaction tubes and is metered to different locations along the catalyst filling via center pipes located in the reaction tubes.
In order to achieve long-term independence from fossil fuels and to reduce carbon dioxide emissions, biomass is increasingly used for providing liquid and gaseous fuels as well as power and heat.
The generation of methane and heat by fermentative processes is a simple and widespread method. However, the generated methane has only a limited concentration and is mostly used only for generating power. It must be further conditioned for use as SNG.
The article “Generating SNG from lignin-rich biomass”, Energie/Wasser-Praxis April 2009, pp 10-16, presents the known modes for utilizing energy from biomass. It is shown that the conversion of biomass to SNG via the intermediate steps of gasification and methanation achieves the highest energy conversion level. Further, in other methods energy in the form of heat and electricity either cannot be stored or can be stored only to a limited extent for flexible commercial use.
Currently available commercial industrial methanation plants and processes were developed for coal gasification. In a majority of these processes, the methanation is carried out at elevated pressure usually between 20 bar and 80 bar and at temperatures between 350 and 500° C. The selected methanation temperatures are deliberately high in order to generate high-pressure steam with the corresponding reaction heat for conjoint generation of electricity. Methanation reactors of this type are challenging from a design perspective and are cost-intensive.
Coal is won in particular regions where large coal deposits are found. In these regions, there is a centralized availability of abundant quantities of coal. Therefore, concepts based on large throughputs and the generation of electrical power by means of high-pressure steam can be carried out economically using coal gasification. However, biomass is not accessible in a centralized manner and, further, has an appreciably lower energy density compared to coal. This leads to plant sizes that are smaller by at least an order of magnitude than those known from coal gasification. Optimal plant sizes for thermochemical conversion of biomass have a fuel performance between 20 MW and 100 MW. Accordingly, the concepts for coal gasification cannot be economically transferred to appreciably smaller biomass gasification plants, which is also why there are no industrial bio-SNG plants at the present time.
Important research work on this topic is currently being carried out, e.g., by the ECN (Energy Research Center of the Netherlands). Studies are being conducted in SNG production through wood gasification and methanation in a fixed bed reactor.
A semicommercial 1 MW SNG plant has been in operation in Güssing, Austria, since 2002. There, it was possible to show an improved generation of SNG from biomass with the AER process in which the biomass is gasified with a two-bed gasifier. In so doing, a circulating solid reactive CO2 adsorbent is used at a low gasification temperature. The Paul Scherrer Institute (PSI) has built a fluidized bed reactor for methanation. In 2009, the plant as a whole produced 100 m3/h SNG of natural gas quality for 250 hours. The plant continues to produce useful heat and electricity. A more detailed description of this and other subjects can be found at www.eee-info.net/cms.
In collaboration with the present applicant, the ZSW is researching a novel methanation process using a salt bath-cooled fixed bed reactor, see Press Release June 2009, www.zsw.de or “Production”, Nos. 24-25, p 15. A corresponding reactor concept was presented at ACHEMA 2009. Two modes for providing synthesis gas as feedstock for methanation were presented. On the one hand, the synthesis gas can be provided from biomass by the AER method. The advantages of the AER method reside in the minimizing of process steps by dispensing with a downstream CO2 separation and dispensing with a CO shift step. On the other hand, previously unused electrical power which is generated from regenerative sources or by conventional power plants at nighttime can be used for hydrogen generation through electrolysis, see Press Release June 2010, www.zsw.de. Stored waste gases from power plants with fossil fuels or waste gases from biogas plants are used as carbon dioxide source.
A reaction tube, the key part of the methanation reactor, was shown schematically with three independent heat carrier circuits with liquid salt. The temperature decreases in the flow direction of the reaction gas. Advantages of this method were the tested reactor type, the scalable design of the reactor, optimal temperature control through adjustable temperature gradients in the range between 550° C. and 250° C. and through high heat capacity of the liquid salt, and operation on the pipe side of up to 20 bar. A synthesis gas having the following composition was used as raw gas for previous tests: 66.5% H2, 8.5% CO, 13.0% CO2 and 12.0% CH4 (%=vol %). A product gas was obtained with 5.9% H2, 8.0% CO2 and 86.1% CH4 (%=vol %). The tests were carried out at temperatures between 550° C. and 250° C. at a pressure of 4 bar and a space velocity of 4,000 Nl/(lcat*h).
The reactor concept presented by the applicant at ACHEMA 2009 allows a more economical methanation of biomass than has been achieved up to this point with other reactor designs. Thus, in adiabatically operated fixed bed reactors with downstream cooling, the reaction is only insufficiently controlled. With fluidized bed reactors, there remains the problem of catalyst attrition and a reaction progress comparable to an ideally mixed continuous stirred-tank reactor without completion of the reaction. However, the production costs are still relatively high, and the exit product gas still does not meet the injection specification for injection into the natural gas grid directly after drying. Therefore, there is still a need for suitable methanation reactors for producing SNG from biomass.